Embryonic stem cells, referred to as ES cells, are derived from the inner cell mass (ICM) of embryos in the blastocyst phase, and can be cultured and maintained in vitro while being kept in an undifferentiated state. ES cells are pluripotent, possessing the capability of developing into any organ or tissue type or, at least potentially, into a complete embryo. For example, ES cells can differentiate and give rise to a succession of mature differentiated cells. Differentiation has been shown in tissue culture and in vivo.
An important application of human ES cells is their use in regenerative medicine, tissue engineering, and cell therapy: the treatment of symptoms, diseases, conditions, and disabilities with ES cell-derived replacement cells and tissues. Many diseases and disorders result from disruption of cellular function or destruction of tissues of the body. A wide spectrum of diseases may be treated based upon both the possession of a population of cells having multi-lineage potential and an understanding of the mechanisms that regulate embryonic cell development. Pluripotent stem cells that are stimulated in vitro to develop into specialized cells offer the possibility of a renewable source of replacement cells and tissue to treat numerous diseases, conditions, and disabilities.
ES cells have been derived from mouse (Evans & Kaufman (1981) Nature 292:154-156; Martin (1981) Proc. Natl. Acad. Sci. USA 78:7634-7639), hamster (Doetschmann, et al. (1999) Dev. Biol. 127:224-227), sheep (Handyside, et al. (1987) Roux's Arch. Dev. Biol. 198:48-55; Notarianni, et al. (1991) J. Reprod. Fertil. 43:255-260), cow (Evans, et al. (1990) Theriogenology 33:125-128), rabbit (Giles, et al. (1993) Mol. Reprod. Dev. 36:130-138), mink (Sukoyan, et al. (1993) Mol. Reprod. Dev. 36:148-158) and pig (Piedrahita, et al. (1988) Theriogenology 29:286; Evans, et al. (1990) supra; Notarianni, et al. (1990) J. Reprod. Fertil. Suppl. 41:51-56). The derivation of human ES cells has also been reported (Thomson, et al. (1998) Science 282:1145-1147; Shamblott, et al. (1998) Proc. Natl. Acad. Sci. USA 95:13726-13731).
Various methods have been described for maintaining ES cell pluripotency and to derive new ES and induced pluripotent stem (iPS) cells (Evans & Kaufman (1981) Nature 292:154-156; Niwa, et al. (1998) Genes Dev. 12:2048-2060; Sato, et al. (2004) Nature Med. 10:55-63; Takahashi & Yamanaka (2006) Cell 126:663-676; Ying, et al. (2003) Cell 115:281-292; Ying, et al. (2008) Nature 453:519-523). For example, screens for molecules that increase cloning efficiency have been described (U.S. Patent Application No. 2008/0171385). In addition, it has been shown that mouse ES cells can remain undifferentiated indefinitely in the presence of an embryonic fibroblast feeder layer. Similarly, it is reported that a feeder layer composed of mitotically inactivated mouse embryonic fibroblasts (MEFs) or other fibroblasts is required for human ES cells to remain in an undifferentiated state (see, e.g., U.S. Pat. No. 6,200,806; Amit, et al. (2000) Dev. Biol. 227:271-78; Odorico, et al. (2001) Stem Cells 19:193-204). However, while mouse ES cells will also remain undifferentiated in the absence of an embryonic fibroblast feeder layer so long as the medium is supplemented with leukemia inhibitory factor (LIF) (Smith, et al. (1988) Nature 336:688-690; Williams, et al. (1988) Nature 336:684-687), human ES cells differentiate or die in the absence of a fibroblast feeder layer, even when the medium is supplemented with LIF (Thomson, et al. (1998) supra).